Monochromator



O. W. PINEO MONOCHROKATOR Feb. 25, 1941.

Filed Aug. 13, 1938 INVENTOR. game/MW. Pnvzo,

ATTORNEY.

30 is reflected from a mirror where the finite mid- While the reflections' ay be effected in y Patented Feh. 25, 1941 v I 4 I UNITED STATES PATENT OFFICE MONOCHROMATOR Orrin Weston Pineo, Milo, Maine, assignor, by mesne assignments, to American Cyanamid Company, New York, N. Y., a corporation of Maine Application August 13, 1938, Serial No. 224,738 5 Claims. (Cl. 240-1) This invention relates to monochromators and 8 rel tively large source of light with corremore particularly to those of the double monos din in ffi ien y b c s f the diifi l y o chromator type. obtaining high intensity light sources of this In the past, two types of double monochromatype. There is also a certain inconvenience due 5 tor have attained practical importance. The .to the fact that the middle slit must be straight 5 first is the conventional type originated by van and the entrance and exit slits therefore curved Cittert in which the light from the light source for t u ate results. passes through an entrance slit, then through a T e d u moncchromator of the p s t prism with two lenses; through a, third l n t vention possesses all of the mechanical advanan intermediate slit and finally through a second t s of e il er reflecting type with the op 10 highly corrected prism and lens system to an l ant es of the van Cittert type and in exit slit. This type of monochromator requires addition, Permits i ngthe same accuracy high grade achromatic lenses and is mechaniw wh ch per p i l D s- A r cally very awkward 'because the travel of the g to e Present invention, light from light light is in the same direction, making an exsource is passed through a prism serving as a trernely long device. This type of monochromarou h filter ,monochrdmator, which y be tor, however, does hav one r al advantage; smaller and less perfect optically, and is then renamely, the deviations of the two prisms com= flect ed a p u l ty of t e the last reflection pensate for each other at a finite middle slit and i ing the sp ctr m n a middle slit which the apex of one prism is imaged on the base of y be slightly curve Af r p s in throuc the other prism so that the length of travel of the the middle Slit. the ly filtered light strikes light through two prisms is uniform over the 8 m p in Such 4! that he -118 f apertur of th system, the first prism is imaged on the base of the sec- The serious mechanical disadvantages of the 0nd and v e versa. and thence passes thr fl van Cittert type double monochromator led to an exit Slit- Here if desired. further reflect on the development of a, compact double monocan be effected if the mechanical-construction Of chromator by Adam Hilger, Ltd of L d in the device makes a change in light direction dewhich light, after passing through the first prism, sirable.

die slit is formed, then sent to the second prism s ita l an I p f to use total internal 0 a d th t an it 111;, A d i i th brefiection in a prism rather than a silvered mirtained which is approximately half as long as the ror and this constitutes the preferred modificavan Citterttype of monochromator,andnshifting 111011 of the invention i y, I prefer to use of the band f light t the exit 1 through the a slightly curved middle slit although the error 3 spectrum is comparatively simple by mechanical introduced by a ai middle t is Small and a movement of the mirror and an associated jaw can be tolerated e e ost economy of conwhich defines the middle slit. The Hllger type struction is desiredof double monochromator is in common use and e monochromatol 0f t e p ese 't invention 40 is standard equipment in recording spectrophoto-' combines the advantages of both Hilger and van meters sold by the General Electric Company. C ttert monochromators without any of the dis- In spite of its many mechanical advantages, th advantages of either. Thus, it is more-compact Hilger'type of double monochromator 'is open to than the Hilger monochromator. In its preferred very serious optical faults. In the first place the form, no silvered mirror is employed, but rather apex of one prism is focussed on the apex of the total internal reflection, so that no discrimina- 5 other. The distance the light travels through tion is effected due to any n-un orm reflecthe glass prisms varies, therefore, across the tion of any spectral colors. Deviations in the prism face, producing a shading of illumination two prisms compensate each other as in the case across the aperture of the instrument in spectral of the van Cittert type of monochromator and regions where the light is somewhat absorbed therefore 8' light Source d e ce s it of 50 by the prisms. An even more serious disadvanminimum size may be used. This results in a tage results from the fact that the deviations by marked increase of efliciency of light utilization;

the mirror are not symmetrical for a finite mid- The middle slit may be slightly curved to permit dle slit and therefore the entrance slit has to be a straight exit slit and source, and is nearly northree times as large as the exit slit, thus requinmal to the beam, as in the van Cittert mono- 55 chromator. A less expensive first prism and associated optics is usable and expensive achromatic collimating lenses which are necessary in the van Cittert type of monochromator can be dispensed with. The fact that the filament image is on the slit and not on the prism is like wise an advantage since the prismis then uni formly illuminated. r

The invention will be described in conjunction with the drawing in which:

Fig. l. is, a diagrammatic illustrationoi the monochromator of the present invention;

Fig. 2 is a side elevation on an enlarged scale partially broken away; and

Fig. 3 is a detailed sectional view taken along the line 3-43 of Fig. 2.

A source of light, commonly a filament 2 enclosed in a bulb l, sends a beam of light through the collimating lenses 3 onto the prism thence through the lens 5 and lens l to the total reflecting prism (3. This prism is carried by the slit 9 which is constrained to follow the line VR by the grooved ball bearing iii running along the knife edge 21!. Lenses 35 are focussed, by lens I and prism 8, onto lens it throughout the traverse of slit 9 by slight rotation of slit 9 about its axis. This rotation is efiected by the grooved ball bearing 6 carried by the light shield is and which runs on the knife edge 20 set slightly out of parallel with knife edge 26.

It will be noted that the light is reflected three times in the prism 8 imaging the spectrum V--R' along the line V-R. Light from the slit 9 passes through the collimator lens H which is a simple lens, through the prism ii, the lens l3 and the exit slit l4 and finally through the lens i5 into the total reflecting prism I6 which folds back the path of light to produce additional compactness. To move the band of light through the spectrum the slit 9 is moved along the line V-R, this being the spectrum image of slit M by elements H, l2 and I3. This motion can be produced by any of the mechanisms commonly employed in monochromators, and a slight rotational motion about the length of the slit is automatically provided so that the lens ll images the lenses 3 exactly onto lens i3 for economy of light. The angles and by which the spectra V-R and V'R' are oblique to the transmitted beam, depend on the constants of the material forming the prisms and lenses. It will be noted that the fllter'prism t is smaller than the prism Hi. This permits greater economy and at the same time makes its spectrum more oblique as is shown 0) to give a little separation between the centerline 22 and the center line 23. When the angle is about half that of 0, the collimator should have one half the focal length of the collimator II and the prism 4 should be about one half as big as prism 92. The collimators v3, a doublet lens, have a focus one third that or 5, giving a. magnification oi three times between the filament 2 and its image in the spectrum V'-R'. To maintain constant frequency range of transmitted light, the slits 9 and I4 must vary in width only about 2 to 1 through the spectrum. Therefore, no entrance aasaoea slit is needed in the filter monochromator and the filament itself can be used for a slit by selecting a proper magnification through proper obliquity of the spectrum V'--R. Slight adjustment of obliquity can be made in assembly by twisting the lens 5 so that the light does not pass through it quite normally. In the filter monochromator this is not harmful and provides means for slight adjustment. The middle slit 9 should be slightly curved so as to use a straight filament 2 and straight exit slit l6 and the light passes through it nearly normally. The curve-- ture is so slight that it is not apparent on the drawing.

By providing for a small entrance slit, which can be formed by the filament itself, it is possible to use motion picture sound track exciter lamps which are excellently suited to this purpose.

Efiiciency is still further increased by the smaller absorption resulting from the smaller size of the filter prism 6. The over-all efiiciency of light utilization of the device is several times that of a Hilger type of monochromator.

While it is an advantage of the present invention that, with available light sources suitably magnified by choice of obliquity of the spectra, it is possible to dispense with physical structure forming an entrance slit, such structure may be included in all cases where the light source is not so chosen as to permit dispensing with entrance slit defined by physical structure.

What I claim is:

1. A double monochromator comprising in optical alignment a narrow band source of light,

1 prismatic dispersing means, a plurality of reflectors, a selector slit, a second prism ic disp means and an exit slit, means including said plurality of reflectors and selector slit between the prismatic dispersing means for imaging the apex of one prism on the base of the other and means for moving the reflectors and selector slit so as to cause the band of light issuing from the exit slit to traverse the spectrum.

2. A double monochromator comprising in optical alignment a narrow band source of light, prismatic dispersing means, prismatic total reflecting means causing a plurality of reflections, a selector slit, a second prismatic dispersing means and an exit slit, means including said prismatic total reflecting means and selector slit between the prismatic dispersing means for imaging the ,apex of one prism on the base of the other and means for moving the reflecting means afnd selector slit so as to cause the band of light issuing from the exit slit to traverse the spectrum.

3. A monochromator according to claim 2 in which the first prism is smaller than the second prism.

4. A monochromator according to claim 2 in which the middle slit is curved.

5. A monochro'mator according to claim 2 in which the light source has the shape of a slit and is at such distance from the first prismatic dispersing means as to obviate the use of a slit between the light source and the first prism.

ORRIN WESTON PINEO. 

